The simplest explanation is that the "hobbits" are Australopithecines. The material evidence is consistent with this and inconsistent with both Homo habilis and Homo erectus. The view that they were later species that "shrunk" is not the simplest theory, and the proposition that smaller primate brains would be more adaptive than larger brains is exceedingly unlikely and needs plausible supporting argument. There are many examples of other primates (orang utans, gibbons, japanese macaques) migrating enormous distances from their place of speciation.